1. Field of the Invention
The present invention relates to a technique for generating a fingerprint image to be used in fingerprint collation processing. In particular, the present invention relates to a technique for using a sensor mechanism to read a series of partial images representing a fingerprint from a finger which conducts slide operation and constructing a general image of the fingerprint from the series of partial images.
2. Description of the Related Art
As a kind of a conventional sensor mechanism for reading a fingerprint image, there is the so-called sweep type fingerprint sensor. The sweep type fingerprint sensor has a sensor surface that is substantially the same in lateral width as a finger and that is shorter in longitudinal length than the whole fingerprint. The sweep type fingerprint sensor is a mechanism which continuously acquires partial images of a fingerprint from a finger which moves in substantially one direction. When constructing a general image of a fingerprint from partial images acquired by the sweep type fingerprint sensor, those partial images are joined on the basis of a movement quantity vector of an input image relative to a preceding image.
In calculating the movement quantity vector, an input image is gradually displaced with an interval corresponding to one pixel, in a predetermined range of the preceding image. The sum total of differences between pixels or lines in mutually corresponding partial images is obtained every displacement as a difference quantity between both images. And a movement quantity vector is calculated on the basis of a displacement at the time when the difference quantity becomes the minimum.
As regards the technique for constructing a general image from partial images of a fingerprint, for example, there are techniques described in JP-A-2003-208620 and JP-A-2003-248820. The technique described in JP-A-2003-208620 includes the steps of setting a region in an input image expected to overlap a preceding image as a noted region, detecting a region that most resembles the noted region from the preceding image, and combining both images so as to make the detected resembling region and the noted region in the input image coincide with each other. The technique described in JP-A-2003-248820 includes the steps of searching for a common region in which feature shapes in two partial images, i.e., a preceding image and a subsequent image, obtaining a movement quantity required when superposing both images in the common region, and joining the two partial images on the basis of the movement quantity.
In a technique for searching for a movement quantity vector which prescribes the disposition of an input image with respect to a preceding image with a pixel pitch, such as the above-described conventional technique, an error is caused by the fact that the actual movement quantity vector component is not necessarily in size an integer times the pixel pitch. Since the errors are accumulated each time joining processing of partial images is conducted, a problem of distortion occurrence in the finally constructed general image is caused. The image distortion caused by the error is remarkable especially in a direction perpendicular to the slide direction of the finger. Its cause is that the movement velocity component of the finger in the perpendicular direction is very small as compared with that in the slide direction. If the error as described above is caused, therefore, distortion of the general image appears remarkably in the perpendicular direction as an influence of the error.
If collation processing is conducted using a fingerprint image having remarkable image distortion, it becomes difficult to obtain a proper collation result. Because the fingerprint image having remarkable image distortion is hard to coincide with a previously registered image, even if the image is a clear image having an area sufficient for the collation. As means for reducing the error, it is considerable to raise the resolution by making the pixel pitch of the fingerprint sensor small. If the means is used, however, the quantity of image information to be processed increases, and consequently the processing capability of the processor and memory capacity capable of coping with the increase become necessary, resulting in a disadvantage of an increased apparatus cost.